Photochromic articles that incorporate photochromic compounds have been known for some time. In particular, many ophthalmic lenses contain photochromic compounds so that when they are exposed to light of a particular wavelength, such as actinic radiation (sunlight), the compound changes from a transparent ground state to a coloured activated state in which the lens is able to filter out at least some of the incident light. The transition from the ground state to the activated state is reversible. Therefore, whilst the photochromic compounds remain exposed to incident light, they remain in the activated state. However, once the source of light is removed, the compounds relax to the ground state in which they are colourless or minimally coloured.
Ophthalmic lenses having a photochromic capacity have been found to be particularly suitable for glasses so that in artificial light the lenses remain transparent, but as a wearer moves into sunlight the lenses darken and reduce the amount of transmitted light. Such lenses are typically formed by including a photochromic compound within the substrate of the lens or on a surface of the lens.
The advent and widespread adoption of ophthalmic lenses formed from plastic materials has meant that new processes have had to be developed to permit photochromic compounds to be used. Typically this is done either by imbibing a photochromic compound directly into the lens substrate, or by coating the lens with a layer containing a photochromic compound.
However, with some known methods of introducing photochromic compounds into plastic articles such as ophthalmic lenses it is difficult to control the extent to which the photochromic compound passes into the article substrate, and/or it is difficult to incorporate sufficient of the photochromic compound into the article substrate or the coating. In addition, some article substrates provide a hostile environment for photochromic compounds because they constrain the transition of the compound from the ground to the activated state and/or they cause fatigue in the photochromic compound.
Additionally, there is a need to provide photochromic ophthalmic lenses in which the transition time between the coloured activated state and the transparent ground state is as short as possible so that the photochromic article reacts as rapidly as possible to any change in lighting conditions.